Comparison of iron and aluminum silicates with the TON topology

A.P. Singh Catalysis Division, National Chemical Laboratory, Pune, India

A crystalline ferrisilicate analog of ZSM-22 was synthesized and characterized by chemical analysis, XRD, framework i.r., e.s.r., d.t.a./d.t.g., ion-exchange, magnetic susceptibility, and catalytic techniques. Scanning electron micrographs and the sorption of water, n-hexane, and cyclohexane show the absence of amorphous material outside as well as inside the unidimensional channels. An increase in the unit cell parameters, shifts in the framework i.r. bands toward lower frequencies, an e.s.r, signal at g = 4.3, magnetic susceptibility data, and a decrease in the d.t.a, peak temperatures (compared to the AI analog) support the presence of iron in the lattice framework of ZSM-22. The material possesses significant ion-exchange capacities and catalytic activity in the conversion of ethylbenzeneo

Keyworda: Synthesis; characterization; catalytic activity; FeZSM-22; ZSM-22; Theta-1; TON

I N T R O D U C T I O N

The replacement of AI by Fe in the framework of various medium- and large-pore zeolites is well established, l ZSM-22 is an orthorhombic high-silica zeolite containing 24 T atoms per unit cell. It has a framework consisting of 5, 6, and 10 rings. The channel system is linear with openings constituted by 10-membered rings. 2 The present work reports the synthesis of such a material wherein A1 s+ has been replaced by Fe s+. The incorporation of iron in the TON framework is confirmed by various physico- chemical techniques. The synthesis of this novel molecular sieve has not been reported so far.

EXPERIMENTAL Synthesis of zeolite

The reaction mixture for the hydrothermai synthe- sis of FeZSM-22 was prepared using silica sol. (SiO~ = 28.9%, A12Os = 0.05%, Na20 = 0.5%, and H 20 = 70.55%), ferric sulfate (GR), potassium hydroxide (AR grade), and 1-ethyl pyridinium bromide. In a typical preparation, 2.18 g KOH in 25 g of doubly distilled water was added slowly under stirring to 20.76 g of silica solution and stirred for 45 min. The resulting mixture was added slowly to a solution of 0.43 g of ferric sulfate in 15 g of doubly distilled water under vigorous stirring. The resulting light pale yellow gel was stirred for 1 h and then a solution of 3.76 g of 1-ethyl pyridinium bromide in 18 g water

Address reprint requests to Dr. Singh at the Catalysis Division, National Chemical Laboratory, Pune 411 008, India. Received 12 October 1991; revised 16 March 1992, accepted 2 April 1992

© 1992 Butterworth-Heinemann

was added to the above gel and stirred for 15 min before transferring it to the autoclave. The resulting gel (pH = 11.80) was heated in a stirred autoclave (100 ml capacity) at 433 K for 4 d under autogeneous pressure. A white crystalline product was obtained that was washed, filtered, and dried at 393 K in air for 12 h. This material was calcined at 713 K for 16 h in a flow of air to remove the organic material and to obtain the potassium form of the molecular sieve. The latter was converted to the NH4 form by treating it with a 8% solution of NH4NOs and NH4OH (pH 7-8) at 333 K. The H form was obtained by calcining the NH4-FeZSM-22 at 743 K for 16 h in a flow of air. The procedure for the synthesis of A1ZSM-22 was the same except that instead of ferric sulfate aluminum sulfate was used. s

CHARACTERIZATION The chemical analyses of A1 and Fe contents in zeolites were carried out by a combination of wet chemical and atomic absorption (Hitachi Z-800) methods. The samples were characterized by the methods published earlier. 1.4-8

Ethylbenzene conversion on the protonic form of the ferrisilicate samples was carried out at atmospher- ic pressure using a downflow tubular silica reactor under the following reaction conditions: amount of catalyst for FeZSM-22 (sample 2) 1.50 g; amount of catalyst for A1ZSM-22 (sample 4) 0.5 g (35-45 mesh); reaction temperature -- 688 K; NJethylbenzene ratio = 4; and ethylbenzene feed = 4 ml/h. The catalyst was activated at 713 K for 2 h with a nitrogen flow rate of 50 cmS/min. Reaction products were analyzed by a gas chromatograph (Shimadzu) using an Apiezon column.

858 ZEOLITES, 1992, Vol 12, September~October

Table 1

Ferrisilicate ZSM-22: A.P. Singh

Synthesis conditions and molar ratios of gels and products of different ZSM-22 zeolites crystallized at 433 K

SiO:z/Fe203 or AI203

Sample In gel

1. FeZSM-22 100 2. FeZSM-22 130 3. FeZSM-22 170 4. AIZSM-22 130

Crystallization pH time Yield

In product (d) Initial Final (%)

78 4 11,50 11.91 94 109 4 11.80 12.03 96 153 4 11.96 12.04 91 104 4 11.85 11.87 96

R E S U L T S A N D D I S C U S S I O N

Chemical composition In the calcined form, the FeZSM-22 (sample 2,

Table 1) has a unit cell composition, in terms of moles of oxides after dehydration, as follows: K0.43 Fe0.4~ Si2~.57 048. The SiO2/Al20 ~ ratio in FeZSM-22 was 700. The molar composition of different FeZSM-22 and AIZSM-22 used in the present study are given in Table 1. The color of the as-synthesized as well as that of the calcined form of FeZSM-22 was white, indicat- ing the absence of brown iron oxide outside the zeolite crystals.

X-ray diffraction The X-ray diff ract ion pat terns for the as-

synthesized FeZSM-22 and A1ZSM-22 are shown in Figure 1, which matched well with those earlier published in the literature for A1ZSM-22. 2'9 The reduced intensity of the diffraction peaks in the case of FeZSM-22 is due to the absorption of X-rays by the heavier iron atoms. The increase in the "d" values for FeZSM-22 (Table 2) is due to the expansion of the framework lattice on incorporation of Fe. A similar phenomena was also found in other zeolites v'8 when A1 was substituted by Fe. These characteristics con- firm the presence of Fe atoms in the framework of ZSM-22.

Scanning electron microscopy Scanning electron micrographs of FeZSM-22 show

the presence of a single phase in the form of needles (0.8-1 ~m long and 0.2-0.3 Ixm thick), which con- firms the absence of amorphous material outside the zeolite crystals. The A1ZSM-22 crystals were also needle-shaped with similar dimensions.

5 20 40

2 e

Figure 1 X-ray powder patterns of as-synthesized (A) AlZSM- 22 and (B) FeZSM-22.

Table 2 "d" values of most significant lines of aiumino- and ferri-ZSM-22 zeolites

AIZSM-22* FeZSM-22*

d (A) ~1o d (A) UIo

10.855 58 10.895 70 8.730 23 8.761 22 6.913 21 6.917 20 5.379 14 5.420 10 4.574 9 4.583 10 4.377 100 4.383 100 3.681 97 3.695 89 3.614 79 3.619 73 3.455 54 3.461 56 2.522 17 2.531 18

aUni t cell vo lumes (A3), FeZSM-22 (78) = 1243.7, FeZSM-22 (109) = 1229.5, FeZSM-22 (153) = 1223.4, and AIZSM-22 (104) = 1220.3, where values in the parentheses refer to the Si02/Fe203 or AlcOa ratios in the product

Thermal analysis Figure 2 shows the thermal behavior of as-

synthesized Fe- and AIZSM-22. The major loss in weight for both the samples occurs in three regions: the first between 373 and 573 K and the other two between 598 and 798 K. The latter are probably due to the decomposition of the organic matter. The weight loss below 573 K is due to loss of water. The AIZSM-22 shows two exothermic peaks (703 and 770 K) due to the oxidative decomposition of the organic matter. In the case of FeZSM-22, these two peaks shift to lower temperatures (680 and 739 K, respectively). The first peak in both cases may be attributed to the

' \A

, /e

i i I i i i = i , 398 59B 798 998 l i 98

TEMP. (K )

Figure 2 D.t.a. (bot tom) and d.t.g. (top) of as-synthesized (A) AIZSM-22 and (B) FeZSM-22.

ZEOLITES, 1992, Vol 12, September~October 859

Ferrisilicate ZSM-22: A.P. Singh

Table 3 Framework i.r. bands (cm -1) of AiZSM-22 and [ FeZSM-22 I AIZSM-22 FeZSM-22 Relative intensity 8

A

1220 1212 m 1084 1080 s =o_- 6

808 806 w ® 786 782 w .~ 641 637 w

u

=4 549 547 m ®

N 484 482 m 462 458 s ~ 2

taJ

s = strong, m = medium, w = weak

oxidative decomposition of occluded organic mate- rial, whereas the latter exothermic peak may be assigned to the oxidative decomposition of organic ions in close interaction with the framework Fe 3+ ions. 7"I° The latter species is held less strongly in FeZSM-22 compared to the A1 analog, causing the observed shift to lower temperature. A similar shift (to lower temperature) in other ferrisilicate zeolite system had also been observed earlier on isomor- phous substitution of A1 by Fe in framework positions.5.1 l-iS

Framework i.r. spectroscopy Most of the framework asymmetric and symmetric

i.r. bands shift to lower wavenumbers (Table 3) when A1 was substituted by the heavier Fe ions in the lattice positions. This observation is similar to our earlier results on other ferrisilicates. 5-8

E.s.r. spectroscopy The e.s.r, spectrum of FeZSM-22 (Figure 3) exhibits

two main signals at g = 4.3 and 2.0.1,14,15 The first is attributed to iron (III) in distorted tetrahedral posi- tions, probably in framework positions. The en- hanced intensity of this signal at liquid nitrogen temperature suggests that it arises from Fe ~+ ions in tetrahedral lattice positions rather than in non- framework positions.

Magnetic susceptibility The magnetic susceptibility was measured between

I I I I I I I I =

t o 0 0 1 5 0 0 2 0 0 0 Z S 0 0 3000 3500 4 0 0 0 4 5 0 0 5000 5 5 0 0 6000

Figure 3 E.s.r. of ferri-ZSM-22 at (A) 297 and (B) 77 K.

• O ¢.

N

0 I I I I I 0 I 2 3 4 5 6

R e o c H o n T i m e / H o u r

Figure 4 Ethylbenzene conversion at 688 K as a function of time on stream (O) HAIZSM-22 and (O) HFeZSM-22.

94 and 298 K for the as-synthesized as well as calcined samples of FeZSM-22 (sample 2). The magnetic moment values for both the samples were found to be 5.7 and 5.4 (Bohr magnetons) at 94 and 298 K, respectively. The relative insensitivity of these values to temperature indicates the absence of a significant amount of iron oxide phases. These observations suggest that Fe 3+ ions are present in a magnetically dilute environment in both the as-synthesized and calcined samples.

Sorption studies FeZSM-22 freely sorbs n-hexane. Cyclohexane

sorption is relatively slow and it is difficult to determine the equilibrium capacity. The sorption capacities of HFeZSM-22 for n-hexane, cyclohexane, and water were 8.8, 1.7, and 4.9%, respectively, at P/Po = 0.5 and 298 K. These sorption values are higher than those for the A1 analog 9 (6.2, 1.3, and 4.4% for n-hexane, cyclohexane, and water, respec- tively) and indicate the absence of occluded amor- phous material within the zeolite channels.

Ion-exchange properties Ion-exchange studies were carried out using the

calcined forms of Fe- (sample 2) and AIZSM-22 (sample 4). Aqueous KNOs and KOH (pH 7-8) was used at 333 K for 2 h. The molar ratio of K+/FeO~ and K+/A10~ - in both FeZSM-22 and AIZSM-22 was found to be 0.92. Tetrahedral framework MO~ groups (M = AI, Fe, etc.) are responsible for the observed ion-exchange properties.

Catalytic properties The catalytic activity in ethylbenzene dispropor-

tionation over HFeZSM-22 and HA1ZSM-22 is shown in Figure 4. HAIZSM-22 is more active than its Fe analog due to the higher strength of the acid sites in the A1 analog, x6 The Fe analog was less active and deactivated at a slower rate than the AI analog. The significant catalytic activity of HFeZSM-22 in ethyl- benzene disproportionation, an acid-catalyzed reac-

860 ZEOLITES, 1992, Vol 12, September~October

tion, provides strong evidence for the presence of most of the iron in the ZSM-22 framework. Protons associated with lattice Fe ~+ ions are the probable active centers.

Ferrisilicate ZSM-22: A.P. Singh

A C K N O W L E D G E M E N T S

The author thanks P. Ratnasamy for helpful discus- sion. This work was partly funded by UNDP.

C O N C L U S I O N

A white crystalline FeZSM-22 was synthesized and the incorporation of framework Fe ~+ was confirmed by the following results:

1. Chemical analysis confirms the presence of iron in the solid FeZSM-22.

2. The "d" values and unit cell volumes (XRD) increase on incorporation of Fe.

3. Scanning electon micrographs and sorption be- havior of FeZSM-22 indicate the absence of any amorphous material outside as well as inside the channels, respectively.

4. The shift in the d.t.a, peaks toward lower temper- atures for FeZSM-22 compared to AIZSM-22 confirmed that the organic matter is held less strongly in the former.

5. On substitution of A1 by Fe, the framework i.r. bands shift to lower frequencies.

6. The magnetic susceptibility data (5.4-5.7 B.M.) confirm that Fe 3+ ions in the sample are in a magnetically dilute environment.

7. FeZSM-22 possesses significant ion-exchange capacity (K+/FeO2 = 0.92) and catalytic activity in ethylbenzene disproportionation.

REFERENCES

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E.W. Zeolites 1985, 5, 349 3 Valyocsik, E.W., US Pat. 4 481 177 (1984) 4 Reddy, J.S., Reddy, K.R., Kumar, R. and Ratnasamy, P.

Zeolites 1991, 11,553 5 Kumar, R. and Ratnasamy, P. J. Catal. 1990, 121, 89 6 Kumar, R. and Thangaraj, A., in Zeolites for the Nineties,

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